1. Field of the Invention
This invention relates to a method of incinerating waste material in a multiple hearth furnace, and to a multiple hearth furnace for carrying out this method. More particularly, the invention relates to a system for controlling the temperatures in the combustion hearth(s) of a multiple hearth furnace, while at the same time controlling the temperature of the afterburner to a nominal temperature to avoid pollution of the atmosphere by the gases exhausted from said afterburner.
2. Description of the Prior Art
Waste materials and particularly sewage sludge, have heretofore been incinerated in multiple hearth furnaces. In the early use of such furnaces, the waste material was simply fed to the uppermost hearth, and air was supplied to the lowermost hearth, and fuel burners were placed on the various hearths as needed for ensuring that combustion took place. The furnace operated to dry the sludge in the uppermost or the next to uppermost hearth, and the thus-dried sludge was passed from hearth to hearth and gradually completely incinerated, the ash being discharged from the lowermost hearth.
In a typical multiple hearth furnace for treating sludge, the furnace is divided into three distinct operating zones:
(1) an upper drying zone defined by a drying hearth in which a major portion of the free water contained in the sludge is evaporated;
(2) an intermediate combustion zone defined by at least one hearth in which the combustible material contained in the sludge is combusted; and
(3) a lower cooling zone defined by a bottom hearth in which the inert solid residue remaining from the combustion process in the combustion zone is cooled by air.
In such a furnace, the solid sludge is introduced into the top of the furnace and descends from one zone into another until it reaches the lowest zone where it is ultimately discharged from a hearth known as the "ash cooling" hearth. Meanwhile, gases from the combustion zones, etc., flow upwards, countercurrent to the downward flow of the solid materials and which gases are treated to remove the malodorous gases and pollutants in an afterburner either located above the hearth defining the drying zone or separate from the main furnace. However, no precise methods have been yet devised to carefully control the temperatures of the individual combustion hearths within carefully controlled limits to prevent, e.g., run away temperatures, and to operate the afterburner within certain limits prescribed by environmental law without the need of adding auxiliary fuel to the afterburner. In respect to the latter point, when the sludge introduced into the drying hearth contains excess water and the combustion gases passing upward over the wet sludge are cooled below the prescribed temperature limits, the gases must ordinarily be further heated in the afterburner by auxiliary fuel to reach the temperature required to comply with environmental laws.
In view of the above, the methods and designs of multiple hearth furnaces used to incinerate sludge have been inefficient in one or more of the above drawbacks previously mentioned.
Recently, there have been attempts made to improve the efficiency of combustion and the design of multiple hearth furnaces. For example, in U.S. Pat. Nos. 4,013,023 and 4,182,246 to Lombana et al., the temperatures in several of the lower hearths have been monitored and the supply of air and fuel to these hearths controlled so as to pyrolyze the materials. By pyrolizing the materials is meant that the waste material is heated in an oxygen deficient atmosphere, i.e., in amounts less than the amount needed to support complete combustion and such operation is carried out in what is called the "pyrolysis mode". In the afterburner, air is introduced to complete the oxidation of the partially oxidized substances which are present in the gases and vapors from the furnace. The air supply to the afterburner is controlled so that at temperatures above a predetermined temperature, the quantity of air introduced is increased with increasing temperatures and is decreased with decreasing temperatures. In other words, the pyrolyzing furnace is caused to operate with a deficiency of air over its operating range, while the afterburner is caused to operate with excess air and the amount of excess air supplied is used to control the operating temperature by cooling or quenching the gases in the afterburner according to these prior art methods.
In U.S. Pat. Nos. 4,046,085 and 4,050,389, a multiple hearth furnace is operated by separately supplying air to the respective hearths to add an oxidant including water vapor, to the fixed carbon zone; or by controlling the amount of air supplied to the respective hearths in response to the temperature on the respective hearths and the temperature of the next higher hearth.
U.S. Pat. No. 3,958,920 shows a multiple hearth furnace in which relatively low temperature gases from the drying zone are recycled to the combustion zone to absorb excess heat. The method of this patent is known as the "Anderson Recycle" and functions by recycling 800.degree. F. moisture-laden gases from the drying hearth back to the combustion hearth to control the temperature. The fan used to recirculate such gases, however, has to handle 800.degree. F. gases with entrained particulate material which is a very severe service. There is also additional electric power required to operate this system.
In all of these recently developed methods of operating a multiple hearth furnace, the purpose has been to control the burning more closely than in the earlier multiple hearth furnaces in order to achieve better incineration of the waste materials.
In such furnaces, however, when sludge is the waste material, it is normally introduced in a form in which it contains an amount of water such that the sludge will not immediately burn. Thus, the sludge is introduced to the upper hearth of the multiple hearth furnace where it is dried by the countercurrent flow of hot flue gases from the combustion hearths below to a sufficiently dry state where it can be burned.
Recent methods have been developed for converting non-autogenous sludge to so-called "autogenous" sludge by a thermal conditioning process. This pretreatment step enables a sufficient quantity of the water to be removed so that the sludge can be supplied to a multiple hearth furnace and incinerated in such a way as to obtain an excess of heat which then can be used for generating steam or the like. Thermally conditioned and dewatered sludge is characterized by low moisture content, high volatile content, and high heating or calorific value; this is as compared to non-autogenous sludge, which has a high moisture content, low volatile content and low heating value. An example of the latter sludge is known as "chemically conditioned sludge".
The introduction of autogenous sludge, such as thermally conditioned sludge, has facilitated the incineration process, making it possible to incinerate the waste material with a minimum of auxiliary fuel needed. Further, combustion with thermally conditioned sludge greatly enhances the energy recovery and steam potential is substantially increased. Improved energy recovery will become increasingly important as energy costs continue to escalate.
While the introduction of such autogenous fuel has been a great boon to the industry, however, it is fraught with certain disadvantages. One of the key disadvantages is that in the combustion of autogenous sludge, it is difficult to control the temperature of the individual combustion hearths within safe operating temperatures because of the high calorific value of such sludge. To counteract this, various methods have been employed to cool down the combustion hearths to avoid thermal stress on the furnace equipment, but most of these methods are largely inefficient. At the same time, because the feed is introduced into the upper drying hearth, it has not been possible to control the temperature of the afterburner to within prescribed environmental conditions without the addition of auxiliary fuel.
The present invention aims at overcoming the disadvantages of the prior art.